The use of effect materials, also known as pearlescent pigments or nacreous pigments, in order to impart a pearlescent luster, metallic luster and/or multi-color effect approaching iridescent, is well-known. Effect materials are composed of a plurality of laminar substrates, each of which is coated with one or more reflecting/transmitting layers of coating materials. Materials of this type derived their characteristics from metal oxides, as described in U.S. Pat. Nos. 3,087,828 and 3,087,829, and a description of their properties can be found in L. M. Greenstein, “Nacreous (Pearlescent) Pigments and Interference Pigments”. Pigment Handbook, Volume I, Properties and Economics, Second Edition, pp. 829-858, John Wiley & Sons, NY 1988.
The unique appearance of effect materials is the result of multiple reflections and transmissions of light. The substrate usually has a refractive index which is different from the coating and usually also has a degree of transparency. The coating is in the form of one or more thin layers which have been deposited on the substrate. If more than one layer is used, the layers are made of materials with different refractive indices. For reflective properties, the outer layer typically has a higher index of refraction compared to the adjacent inner layers. Pearlescent luster is derived from specular reflection from the surfaces that are essentially parallel to each other.
One aspect of the coating on the substrate is that it must be smooth and uniform in order to achieve the optimum pearlescent appearance. The reason is that if an irregular surface is formed, light scattering occurs and the coated substrate may not function effectively as an effect pigment.
In addition, the first coating should adhere strongly to the adjacent coating or substrate, or else the coating may become separated during processing, resulting in considerable breakage and loss of luster. Particles which do not become attached to the substrate during preparation of the coatings on the substrate or which are the result of separation cause light scattering and impart opacity to the pigment. When there are too many of such small particles, the pearlescent appearance can be reduced or lost.
U.S. Pat. No. 5,171,363 discloses a multilayer structure comprising alternate layers of a material having a low refractive index of 1.35 to 1.65 and a material having a high refractive index of 1.7 to 2.4. One example is silicon dioxide (SiO2; refractive index of 1.5) and titanium dioxide (TiO2; refractive index of 2.7). The layers are formed by vacuum coating, electron beam, or sputtering. The resulting optically variable flake is used to make optically variable ink.
Japanese Patent Application No. 7-246366 discloses alternating layers of SiO2 and TiO2 sputtered or vaporized to form a pearlescent material for paint. The layers are applied to a substrate such as glass.
U.S. Pat. No. 4,879,140 discloses the use of plasma enhanced chemical vapor deposition to deposit multilayer films comprising alternating layers of SiO2 and silica to a total of 30 layers for use as an interference filter. The patent also discloses deposition of alternating layers of SiO2 and TiO2 to a total of 31 layers having a total thickness of approximately 2 microns.
U.S. Patent Application Serial No. 20070029561 discloses an omni-directional reflector having a transparent conductive low-index layer formed of conductive nanorods and a light emitting diode utilizing the omni-directional reflector are provided. The omni-directional reflector includes a transparent conductive low-index layer formed of conductive nanorods and a reflective layer formed of a metal.
Despite the advances in the art, a need exists to produce unique optical materials in the form of thin films by tailoring the refractive index of a variety of materials.
In many applications, the high density of certain materials, for example, iron oxide and titanium dioxide, are problematic in formulations; hence, there is a need for low density thin film based materials where a material such as iron oxide has reduced density.